Generally the present invention relates to a refrigeration system. More specifically, but not exclusively, the present invention is directed to systems and methods for controlling coolant flow in refrigeration systems and associated components.
A typical refrigeration system includes a compressor pumping a two-phase coolant through a circuit that includes a condenser, a metering device, such as an expansion valve, and an evaporator coil. It is important to control coolant flow through the circuit to operate the system efficiently. Further, and possibly of greater importance, controlling the coolant flow minimizes the risks for damaging the components of the system. The metering device can be used to control the flow of coolant through the evaporator coil. Too little coolant flow does not withdraw sufficient heat from the surroundings. Too much coolant flow can increase the amount of liquid coolant in the suction line, which in turn can flood the compressor causing catastrophic pump failure. One method for monitoring the coolant flow, or specifically liquid coolant, is to determine the coolant superheat level. The amount of superheat is the temperature level of the coolant above its saturation temperature level at a given pressure. Typically a system""s manufacturer specifies a lower limit for the coolant superheat value to avoid flooding the compressor.
The superheat level is difficult to accurately measure and even more difficult to accurately control. Few systems actually measure true superheat. The level of superheat is typically determined by measuring the temperature drop across the evaporator coil. This can be accomplished by measuring the temperature level of the coolant immediately after the metering device and again at the outlet of the evaporator coil. Opening and closing the metering device to control the superheat level of the coolant can vary the coolant flow through the evaporator coil. For example, opening the metering device increases the amount of coolant flow and reduces the superheat value, assuming the coolant withdraws a constant amount of heat in the evaporator coil. Conversely, closing the metering device reduces the coolant flow and concomitantly increases the superheat level, again, assuming the coolant withdraws a constant amount of heat in the evaporator coil.
In addition to monitoring and controlling the coolant flow, the system should be properly maintained to ensure efficient operation. For example, the evaporator coils should be regularly defrosted to eliminate ice buildup on the coils. This allows a more efficient heat transfer from the controlled space to the coolant. During the defrost period, heat is applied to the coils. This period represents additional energy drain on the system and xe2x80x9cdowntimexe2x80x9d because the system is not actually cooling the controlled space. Consequently, it would be desirable to only defrost the coils when necessary to efficiently chill the controlled space.
Accurate control of the refrigeration system is difficult because a refrigeration system is a dynamic system. The load on the system, i.e. the amount of heat absorbed or withdrawn by the coolant to maintain its desired temperature level in a controlled space, can frequently change. Additionally, defrost cycles interrupt operation of the system. Consequently, the load on the system can vary dramatically depending upon many factors such as the size of controlled space, the defrost cycle, system start up, and the number, size, and condition of products placed into or taken out of the controlled space.
However, despite the difficulties noted above, refrigeration systems can be operated more efficiently with less equipment failure, including fewer incidences of flooded starts and/or compressor failure, by proper maintenance and accurately controlling coolant flow. Thus, in light of the above-described problems, there is a continuing need for advancements in the relevant field, including improved methods and devices relating to controlling refrigeration systems. The present invention is such advancement and provides a wide variety of benefits and advantages.
The present invention relates to devices and methods for controlling refrigerant systems. Various aspects of the invention are novel, non-obvious, and provide various advantages. Certain forms and features, included in the invention disclosed herein, are described briefly as follows.
In one form, the present invention provides a coolant system that includes a coolant circuit having a low pressure portion and a high pressure portion, and comprises, in series, a compressor circulating coolant through the circuit; a user-selectable metering valve controlling fluid flow through a low pressure portion of the circuit; an evaporator coil, a first sensor downstream of the metering valve providing a first signal representative of a first pressure in the circuit; and a second sensor proximate to the compressor providing a second signal representative of a first coolant temperature in the low pressure portion of the circuit; and a controller operably connected to the user-selectable value. The controller is adapted to receive the first signal and the second signal, and in response generates a third signal controlling the user-selectable valve.
In another form, the present invention provides a refrigeration system including a coolant circuit for a coolant, a compressor, a metering device, an evaporator coil, and an evaporator fan. The system comprises first sensor evaluating a condition representative of a load on the fan or the amount of work performed by the fan; the first sensor can generate a first signal representative of the amount of work to the controller. The system also includes a controller operably connected to the fan and adapted to initiate a defrost sequence.
The controller also can be operably coupled to one or more memory storage devices that have instructions for initiating a plurality of predetermined defrost periods or a defrost sequence. In response to a signal from the first sensor, the controller can select one of the predetermined defrost sequences to perform. Alternatively, the controller can accumulate the total amount of time of the defrost time in the defrost sequence, and, in response to the signal form the first sensor, the process can adjust the amount of defrost time over a specified time period or defrost sequence accordingly. Additionally, the controller can evaluate the superheat level of the coolant and compare that level with a target superheat value. In response to this comparison, the controller can adjust the output of the metering device. Alternatively, the controller can evaluate the coolant pressure in the system and compare that pressure level to a target pressure value. In response to the results of that comparison, the controller can adjust the output level of the metering device.
In another form, the present invention provides a method of controlling a refrigeration system that comprises a compressor, a coolant in a coolant circuit, a metering device, an evaporator coil, and a fan positioned near the evaporator coil. The method comprises defrosting the evaporator coil for a selected defrost period; determining the difference in the load or amount of work performed by the fan measured before and after the selected defrost period; and adjusting one or more of a defrost sequence, a defrost delay period, a defrost time period, or an amount of heat transferred to said evaporator coil in response to the work difference.
In still yet another form, the present invention provides a method of controlling a refrigeration system that includes a compressor, a coolant in a coolant circuit, a metering device, an evaporator coil, and a fan proximate to the evaporator coil. The method comprises determining a superheat level of the coolant in the coolant circuit; comparing that superheat level of the coolant to a target superheat level; and adjusting a first output level of the metering device in response to said comparing by reducing the first output level by an amount equal to the sum of: a first value equal or less than about 62% of the relative flow when the coolant superheat level of the coolant is less than or equal to about one half of a target superheat value. In preferred embodiments, the controller adjusts the metering device to have a subsequent output equal to about Yn+1 times the first output value where Y is selected to be between about 0.3 and about 0.62 and wherein n is selected to equal an amount, in degrees Fahrenheit, that the coolant superheat level of the coolant is below about one half of the target superheat value.